Devices employing low temperature superconductors must be refrigerated to bring the superconducting material below the critical temperature at which the material becomes superconducting. For many superconductors (low temperature superconductors) this critical temperature is extremely low, often in the vicinity of the temperature of liquid helium. Other superconductors are high temperature superconductors (HTS) and must be maintained at less than about 82K (for presently available HTS materials). In either case the difference between room temperature (295K) and the temperature of the superconducting device can be very large. Also, the temperature difference between a region containing a low temperature superconductor and a region containing an HTS may be very large. As a result of this large temperature difference the energy lost through even a small thermally conductive area in the electrical lead transition between these regions can be significant.
In some applications a low temperature superconductor is maintained at a desired low temperature (less than 2.17K) by means of a superfluid helium (He II) bath at 1.8K in a first region. An adjacent region is maintained at 4.2K in a liquid helium (He I) bath. The length to cross-sectional area of this region is a trade off between lower heat conduction during operation when the lead is superconducting and less ohmic heating when the lead becomes a normal conductor due to an energy disturbance. It is advantageous to minimize any heat leak to 1.8K as removal of one watt of heat leak requires about 1000 watts of refrigeration power.
Heat transfer may occur through electrical leads (transition leads) which bridge the room temperature environment of a power supply and the very low temperature environment of a superconducting device. The leads are provided to supply current to the superconducting device from outside the refrigerated device, and often carry very high currents. Unfortunately, normal conductors of electricity--especially those designed for high current loads--are generally excellent thermal conductors. Also, when large amounts of current flow through a normal conductor, resistive heat losses are generated. Consequently, when normal conducting leads are connected to the refrigerated superconducting device, they can place a significant load on the refrigerating equipment for the superconducting device. The added power required to refrigerate the device can significantly degrade the total power efficiency of the system.
In many superconductor applications, for example in the superconducting magnets used in nuclear magnetic resonance diagnostic machines and particle accelerators, a current is only applied to the device during the charge and discharge of the device. Since the time devoted to charging and discharging the device can be very small in proportion to the time when the device need not be electrically connected to the exterior (in a standby mode), it is common practice to disconnect the electric leads into the device throughout the standby period.
Unfortunately, the mechanical joints in detachable leads reduce the reliability of the system. The mechanical joints are exposed to extreme temperature variations and a harsh environment which can lead to a connection failure that could be disastrous to the functioning of the superconducting device.
Another system, described in U.S. patent application Ser. No. 07/422,642, by Hilal, entitled Low Heat Loss Lead Interface now U.S. Pat. No. 5,057,645, includes a disk separating two regions of different temperatures. Within the disk, there is a superconductive winding. One side of the disk is connected to a normal conducting lead in a relatively warm region. The other side of the disk is connected to a superconducting lead in a relatively cold region. The disk is metal and has a channel formed therein to allow a coolant to pass through it. Another type of lead for connecting a superconductor in a first temperature region to second temperature region was described in the 1990 Applied Superconductivity Conference, Snow Mass Village, Colorado, Sep. 24-28, 1990. The leads described therein were designed to connect low temperature superconductors to high temperature superconductors (HTS), and the lower part of the conventional copper leads were replaced with HTS tapes or bars. The HTS tapes or bars provide no I.sup.2 R loss and very little heat conduction. However, these kinds of leads are unstable. If the HTS goes normal (i.e. the temperature rises to the point that the HTS stops superconducting), the current has no copper shunt to flow through because the copper shunt is removed to reduce the heat load by conduction, resulting in excessive heating with quickly rising temperature.